1. Field of the Invention
The present invention relates to an endoscope system having the function of imaging an oxygen saturation level of blood, a processor device of the system, and a method for displaying the oxygen saturation level.
2. Description Related to the Prior Art
Endoscopy is widely available as a minimally invasive diagnostic and treatment procedure. The endoscopy uses an endoscope system that has an electronic endoscope, a light source device, a processor device, and a monitor. Recently, there is known the endoscope system that has not only a normal mode of observing the inside of a body cavity or lumen under white light (normal light), but also a special mode using light (special light) in a specific narrow wavelength band to facilitate finding out a lesion.
In the normal mode, the normal light such as xenon light in a wide wavelength band is applied from the electronic endoscope inserted into the body cavity to an internal body surface, and a color image sensor captures the light reflected from the body surface. A color image obtained by the image sensor is subjected to image processing in the processor device, and is displayed on the monitor. In another technique, as described in Japanese Patent Laid-Open Publication Nos. 2001-218217 and 2002-085344, the normal light is separated into three color light beams of R, G, and B, and the three color light beams are sequentially applied to the internal body surface. The reflected light beams from the body surface are captured by a three frame sequential method to obtain three frame images. Then, the frame images are merged into a single full color image.
In the special mode, a blood vessel pattern obtaining technique is known in which a blood vessel in a specific depth is emphasized using the special light having a wavelength at which hemoglobin has a high light absorption coefficient. Also, as described in U.S. Pat. No. 5,956,416 and US Patent Application Publication No. 2011/237884, there is known an oxygen saturation level obtaining technique. In this technique, a plurality of special light beams that have different wavelengths including a wavelength at which the light absorption coefficient much differs between oxygenated hemoglobin and deoxygenated hemoglobin are sequentially applied to the internal body surface. The reflected light beams therefrom are captured to obtain a plurality of frame images. The oxygen saturation level is calculated from the plurality of frame images. The above techniques facilitate finding out a lesion such as cancer, which is hard to spot under the normal light.
As described above, in image processing or calculation using a plurality of frame images, difference in imaging timing sometimes causes positional displacement among the plurality of frame images in either of the two modes. In the three frame sequential method, the positional displacement among the frame images manifests itself as color displacement in the full color image. The faster the image sensor contained in a head assembly of the electronic endoscope moves, the more likely the positional and color displacement occurs. For this reason, according to the Japanese Patent Laid-Open Publication No. 2001-218217, if the color displacement caused by the positional displacement among the frame images is beyond an allowable range, a portion of the displacement is masked to visually indicate that the portion has low reliability of the oxygen saturation level. In the US Patent Application Publication No. 2011/237884, registration among the frame images is carried out with respect to the position of a blood vessel to eliminate the positional displacement.
According to the Japanese Patent Laid-Open Publication No. 2002-085344, when the three color freeze frame images of R, G, and B are stored to a memory in response to a freeze command, a movement amount of the head assembly is calculated from the displacement among the frame images of the same color obtained upon the command and before and after the command, and the registration among the three frame images is performed based on the movement amount. Similarly, according to the U.S. Pat. No. 5,956,416, in the correction of the positional displacement among the color frame images of R, G, and B, the frame images of the same color obtained upon the command and before and after the command are converted into binary images. The freeze frame images are made in register based on displacement in a barycenter among the binary images, to eliminate the color displacement in the full color image.
In several years, expectations on diagnosis using the oxygen saturation level of blood are rising. This is because diagnosis using the blood vessel pattern requires a doctor to have adequate knowledge about the blood vessel pattern specific to the cancer, but in the diagnosis using the oxygen saturation level, a hypoxic region being a sign of the cancer is artificially colored on the monitor. Therefore, since the doctor can grasp the lesion at the sight of the monitor, much experience and knowledge are not required of the doctor as compared with the diagnosis using the blood vessel pattern.
In the diagnosis using the oxygen saturation level, the hypoxic region and a hyperoxic region surrounding the hypoxic region are defined as the lesion, so it is required to clearly display a boundary between the hypoxic region and the hyperoxic region by accurately calculating the oxygen saturation level. Thus, it is desirable to calculate the oxygen saturation level with high accuracy. If the calculation accuracy deteriorates due to the positional displacement among the frame images caused by difference in the imaging timing, it is desired to indicate the deterioration in the calculation accuracy. In the Japanese Patent Laid-Open Publication No. 2001-218217, the portion of the color displacement is masked as an invalid area to visually indicate that the portion has low calculation accuracy. However, even in a valid area having no color displacement, the calculation accuracy may be degraded due to another factor such as blood volume. On the other hand, according to the US Patent Application Publication No. 2011/237884, although the registration among the frame images improves the calculation accuracy of the oxygen saturation level, the calculation accuracy is not indicated on the monitor. Even if the image registration is performed inadequately, a calculation result with low accuracy is displayed on the monitor without any notice.